The present invention relates to apparatus for converting an AC line voltage of low frequency into a high frequency output AC voltage and, more particularly, to a low cost miniaturized electronic ballast circuit for operation of electric discharge lamps.
In prior art electronic ballast circuits, a separate power factor correction circuit is employed in order to obtain a high power factor. FIG. 1 depicts a conventional half-bridge electronic ballast where a boost converter is used for power factor correction. An EMI (electromagnetic interference) filter is coupled to the terminals of a low frequency AC line voltage, for example, 50 Hz or 60 Hz, and is used to filter out the high frequency noise generated by the high frequency operation of a boost converter and a half-bridge DC/AC inverter coupled in cascade to form the electronic ballast circuit. The line voltage is rectified by a full bridge rectifier D1-D4 to produce a pulsatory DC voltage. In the boost converter stage, which is coupled to the output of the rectifier circuit, the current flow through the inductor L is regulated based upon a reference current generated from the rectified line voltage by means of the control circuit A so that the current waveform is shaped to be the same as and to be in phase with the rectified voltage waveform. This is done by controlling the ON duty ratio and/or the frequency of the MOSFET switch Q.
The boost converter receives the pulsating DC voltage from the bridge rectifier D1-D4. When the switching transistor Q is turned on, a current flows from the rectifier bridge through the inductor L and the transistor Q so that electromagnetic energy is stored in the electromagnetic field of the inductor. When the transistor Q is switched off, the electromagnetic energy in the inductor and energy from the line are; transferred to the storage capacitors Ce1 and Ce2 of the boost converter as a result of a current that flows via the inductor L and the blocking diode D. This represents the conventional mode of operation of a boost converter.
The output of the boost converter is a DC voltage across capacitors Ce1 and Ce2 coarsely regulated by the control circuit A. This DC voltage is then inverted into a high frequency AC voltage by the high frequency half-bridge DC/AC inverter coupled to the output of the boost converter so that a regulated output power can be obtained for the load. Since the input power of the boost converter possesses a low frequency component (100 Hz or 120 Hz) and the output power of the half-bridge DC/AC inverter is a regulated high frequency power, an energy storage capacitor, here shown as capacitors Ce1 and Ce2, is placed between the boost converter stage and the half-bridge inverter stage so as to balance the input power and the output power.
Depending on the manner in which the current flow through the inductor L is controlled during each switching period, the operation mode of the boost converter can be classified into two categories, i.e. Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). In CCM, the high frequency ripple of the current through the inductor L can be made small so that the stress on the EMI filter can be reduced. A drawback of this control method is that the duty ratio of the switching MOSFET Q has to vary with the rectified line voltage, thus resulting in a more complex control circuit. In the DCM, the peak values of the current through the inductor L automatically follow the waveform of the rectified line voltage if the on time of the MOSFET is constant. Therefore, the control circuit can be greatly simplified. This advantage is the main reason why DCM is usually adopted in the boost converter stage for power factor correction in low-power applications.
An important disadvantage of the electronic ballast circuit of FIG. 1 is that the requirement of a separate power factor correction (PFC) stage and a separate DC/AC inverter stage increases the cost and size of the overall ballast device. One prior art attempt to reduce the circuit complexity is described in U.S. Pat. No. 4,564,897 (Jan. 14, 1986) in which a smoothing (i.e. power factor correction) circuit and an inverter circuit share a common switching element and the control circuit thereof. This U.S. patent is hereby incorporated by reference and discloses a power supply which employs a relatively small inductor while providing a relatively high power factor. The high frequency AC output voltage has a relatively low line-frequency ripple component which makes it suitable for operation of a discharge lamp. However, this power supply has a serious problem in that it is very difficult to regulate the inverter output at a desired stable level in the event of a variation in the input AC voltage or in the case of varying load requirements.
U.S. Pat. No. 5,182,702 (Jan. 26, 1993) describes an inverter device which attempts to solve some of the disadvantages of the prior art by providing a simpler control circuit for the overall system. This inverter device includes a full wave rectifier supplying a pulsating DC voltage from an input AC voltage to a smoothing (power factor correction) circuit. The power factor correction circuit includes an inductor, a smoothing capacitor and a switching element for chopping the pulsating DC voltage through the inductor into a smooth DC voltage at the capacitor. An inverter is provided which has a switching element common to the power factor correction circuit and operative to switch the DC voltage to apply a high frequency voltage to a load circuit including a load, an inductance and a capacitor. The inductance and capacitor define a resonant circuit providing the load with an oscillating current composed of first and second opposite flowing currents. The oscillating current flows for a nominal on-time determined by a circuit constant of the resonant circuit. A controller detects the termination of the second current and excites the common switching element at a time dependent thereon so as to start the flow of the first current. The controller includes a timer for separately controlling the actual on-time for the flow of the first current within the nominal on-time. This makes it possible to regulate the accumulated DC voltage on the smoothing capacitor at a desired level by controlling the actual on-time period of the common switching element. This circuit also has certain disadvantages which limits its use in practical applications.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a low cost miniaturized electronic ballast circuit or high frequency AC/AC converter having improved operating characteristics and which is not subject to the limitations of the prior art.
It is a further object of the invention to provide a simple and compact high frequency AC/AC converter circuit having a high power factor and low current distortion.
It is another object of the invention to provide an electronic ballast circuit which requires only one control circuit and fewer switching transistors than the electronic ballast circuit shown in FIG. 1.
It is a further object of the invention to provide an electronic ballast circuit with a superior lamp ignition characteristic to that of the prior art inverter device.
A still further object of the invention is to provide an electronic ballast circuit or high frequency AC/AC converter that produces a more sinusoidal waveform and substantially reduces the level of voltage spikes generated across the diodes of the full-wave rectifier at the input of the electronic ballast circuit or high frequency converter.
Another object is to provide an electronic ballast circuit including a high frequency DC/AC inverter part utilizing a resonant LC circuit that improves the circuit performance and makes for greater flexibility in circuit design.
It is a further object of the invention to combine the functions of the PFC boost converter and the high frequency DC/AC inverter into a single combination AC/AC inverter with input power factor correction.
It is yet another object of the invention to use a pulse width modulation (PWM) technique which results in a very simple control circuit for the electronic ballast.
The foregoing and other objects and advantages of the invention are achieved, inter alia, by combining the separate functions of the input PFC boost converter and the high frequency DC/AC inverter into a single stage high frequency AC/AC inverter with power factor correction thereby providing power factor correction and a high frequency DC/AC inverter operating characteristic.
Since there are two separate stages in the prior art system shown in FIG. 1, two separate control circuits are required. The combination circuit of the present invention combines both the power factor correction stage and the high frequency inverting stage into a single stage so that one power stage and its corresponding control circuitry are no longer required and the circuit cost is thereby reduced.
In a preferred embodiment of the invention, a pair of input terminals for connection to a 60 Hz AC line voltage or the like is connected to a pair of input terminals of the high frequency AC/AC converter with power factor correction via an electromagnetic interference (EMI) filter and a diode bridge rectifier circuit. A series circuit comprising a first inductor, a diode, first and second capacitors, a second inductor and the primary winding of an output transformer is connected to the input terminals of the high frequency converter. The second input terminal is connected to a common line of the high frequency converter. A first switching field effect transistor (FET) is connected to a first junction point between the diode and the first capacitor and to the common line. A second switching FET is connected to a second junction point between the first and second capacitors and to the common line. The transformer secondary winding is coupled to the load (e.g. a discharge lamp) and a single control circuit receives a feedback signal from the load circuit to control the switching of the first and second field effect transistors at a high frequency. A third capacitor is coupled across the transformer secondary winding and forms an LC resonant circuit with the second inductor.
Two distinct control techniques can be employed in accordance with the present invention. One is constant duty ratio control and the other is duty ratio sweeping control. These control techniques will be described in greater detail in connection with the detailed description of the drawings .